This invention relates to methods for fabricating multilayer printed circuit boards; and more particularly, to methods of fabricating electrical contacts between the layers. In cross section, a typical multilayer circuit board may have an outer copper layer that is followed by an epoxy layer, an inner copper layer, another epoxy layer, and an outer copper layer. And basically, the problem that is dealt with in this disclosure is how to fabricate holes through all of those layers in a manner that allows reliable electrical connections to be made between the inner and outer copper layers.
In the prior art, those contacts were made by first mechanically drilling holes of various diameters (such as 60 mils for example) through all of the layers. This mechanical drilling however, forms an epoxy smear on the walls of the holes. Sulfuric acid has been used to dissolve this smear. But as the board is subsequently rinsed in water to remove the sulfuric acid, some of the smear comes out of solution and precipitates back on the walls of the holes. This precipitate prevents reliable electrical contacts from being made between the copper layers.
Various chemicals have been used in the past in an attempt to remove this precipitate. These chemicals include:
Ethylene Glycol PA0 2(2-phenoxyethoxy) ethanol PA0 Isobatoxy-2-propanol PA0 NaOH PA0 Methyl Ethyl Ketone PA0 Methylene Chloride PA0 1,1,1-trichloroethene PA0 Toluene PA0 Acetone PA0 Sodium Chlorite PA0 Butyl Carbitol PA0 Acetates
However, all of those chemicals have been found unsatisfactory in that poor electrical contact with the conductive layers still intermittently occur.
Therefore, a primary object of the invention is to provide an improved method of fabricating multilayer printed circuit boards.
Another object of the invention is to provide a method of fabricating sound and reliable electrical contacts between the conductive layers of a multilayer printed circuit board.